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I would like to double a packed BCD number.

eg. 1234 decimal can be represented thusly in 32 bits:
$000004D2 hexadecimal conversion
$01020304 BCD (each digit stored in a byte)
or $00001234 packed BCD (each digit stored in a nibble)

A complete routine for adding two packed BCD numbers is given here http://www.asmcommunity.net/board/viewtopic.php?t=13215

What I want to do now is multiply a packed BCD number by 2. The following code works (and incorporates carry in & out). It simplifies one aspect of the addition algorithm: when binary number being added to itself, normally just shift....

Can it be sped up any? Thanks.

     mov ([esi+mda], eax);

     add ($33333333, eax);

     mov (eax, edx);

     xor (-1, edx);

     and ($88888888, edx);

     shr (1, ebx);              // carry

     rcl (1, eax);

     rcl (1, ebx);              // carry

     sub (edx, eax);

     shr (2, edx);

     add (edx, eax);

as '0x33' HexMark, it's belong C (or other HLL) .

all most Asms' HexMark, has 2, "33h" & "$33" ; 33h in asm source, written by Asm programmer; $33 in online disassembler for display.

 

    mov ecx, [esi+mda] 

    lea eax, [ecx+33333333h] 

    and eax, 88888888h

    shr eax, 1 

    add ecx, eax 

    shr eax, 1 

    add eax, ecx

Thanks guys. But it does not work. With input 123456h, output eax=1234BCh ecx=12349Ah. Something should be 246912h. Please also check out at the other links. I like the brivity of your routine though, so I'm looking at it further. Do you have a source for it or stuff on packed BCD arithmetic? Thanks.

That is great. Four instructions less than mine!!! How did you do it? BTW one thing missing though - how do we get the carry please in case of doubling eight digit numbers?

Another question. How would you extend this to a generic add for packed BCD please? Also with carry?

Do you have any other links on the topic please? Thanks.

I got it. This handles the carry...

    mov ecx, [esi+mda]

    lea eax, [ecx+33333333h]

    and eax, 88888888h 

    lea ecx, [ecx*2+ebx]

;    add ecx,ecx

    sets bl

    add ecx,eax

    shr eax,2 

    sub ecx,eax

Still, how would you extend this to generic packed BCD add please. Thanks.

I combined the two techniques above to produce:

mov ecx, [esi+mda]

lea eax,[ecx+33333333h] 

adc ecx,ecx 

rcr eax,2 

and eax,22222222h 

lea eax,[eax*2+eax] 

add ecx,eax 

shl eax,2

However, it is one quarter the speed of

mov ecx, [esi+mda] 

lea eax,[ecx+33333333h] 

adc ecx,ecx 

and eax,88888888h 

add ecx,eax 

shr eax,2 

sub ecx,eax 

shl eax,3

and

mov ecx, [esi+mda] 

lea eax, [ecx+33333333h] 

and eax, 88888888h 

lea ecx, [ecx*2+ebx] 

; add ecx,ecx 

sets bl 

add ecx,eax 

shr eax,2 

sub ecx,eax

Apparently the flags dependency after adc slows down the rcr - and we don't even use this carry!!! Changing rcr to shr brings it almost up to speed.

mov ecx, [esi+mda]

lea eax,[ecx+33333333h] 

adc ecx,ecx 

shr eax,2 

and eax,22222222h 

lea eax,[eax*2+eax] 

add ecx,eax 

shl eax,2

Consider the four routines above (Sep 8 ) to be labelled a, b, c, d. They rank as follows:

1st place <b> (Sephiroth3 - Sep 7 with carry in carry flag) and <c> (V Coder - Sep 6 with carry added in ebx & sets bl after Sephiroth3 - Sep 6) calculation in 5 seconds (both approx 5.75 seconds but truncated to 5). Total with startup and save=8.5 seconds
2nd place <d> (V Coder - Sep 8 with carry in carry flag after Sephiroth3 - Sep 7) calculation in 7 seconds
3th place <not posted> (V Coder - a variation of the initial Sep 2 code using setc bl) calculation in 7 seconds
4rd place <initial code> (V Coder Sep 2) calculation in 10 seconds
5th place <a> (V Coder - Sep 8 with carry in carry flag after Sephiroth3 - Sep 7) calculation in 25 seconds

Total time with startup & save measured with Randy's howlong.

With twice as large terminal dataset, b & c completed the calculation in 23 seconds (4 times as long), and the initial code took 44 seconds. b took 25.7 seconds and c 26.1 seconds according to howlong. b is therefore just under twice as fast as the original routine. Furthermore, all things being equal, b should save a minute every hour over c.